In many radio and radar systems, the signals of interest will occupy only one sideband of the carrier frequency, while the other sideband contains unwanted signals such as interference, clutter, and noise. Even in systems which utilize both sidebands of the carrier for the desired signals, there may be a need to discriminate signals between the two sidebands. In either of these cases, the receiver must be capable of unambiguously resolving signal components which reside in the upper and lower sidebands of the carrier. To do this, the receiver must sample the intermediate frequency (IF) or baseband signal to produce both in-phase (I) and quadrature (Q) components. If the signal is sampled with only one phase (yielding a set of real-valued rather than complex-valued samples), then the measured IF or baseband spectrum will consist of one sideband superimposed with the mirror image of the other sideband, reflected across fIF.
Obtaining both I and Q samples in the receiver traditionally requires an analog I/Q architecture, consisting of two full signal chains, including two mixers, two sets of filtering and amplification stages, and two analog-to-digital converters (ADCs) for each receiver channel. An alternative is direct IF sampling which uses only one signal chain (one mixer, one set of filter/amp. stages, one ADC) per channel, but requires a higher performance ADC and involves some digital signal processing to be performed in order to recover the I and Q samples. In either case, designing a receiver with I/Q detection usually requires significantly more hardware, software, or both, which in turn increases the size, weight, power consumption, and cost of the receiver.